The present invention relates to a cam follower for constituting a valve mechanism of an engine.
In recent years, a valve mechanism for transmitting the motion of a cam, which is fixed on a cam shaft to rotate together with the crankshaft of an engine, to a valve is reduced in frictional loss to a low level by assembling a cam follower into the valve mechanism thereby to change the friction at the running time of the valve mechanism from a sliding friction to a rolling friction.
Here, of the engine parts, lubricating property of the cam portion is severe, and its contact face is called the boundary lubricant area. In the cam follower with the roller to be used under this condition, the roller basically makes a rolling contact with the cam, but the pure rolling contact is impossible because of the change in the rotational speed of the roller due to the cam shape or the abrupt fluctuation of the load acting on the roller. Thus, the cam follower makes a rolling contact accompanied by the slip. In relation to the lubricating conditions (on quantity and temperature of oil and on a foreign substance) and the surface roughness of the mating cam, the peeling phenomenon arises on the outer rolling face of the roller, thus raising a problem that the cam follower has a short lifetime.
In order to improve the durability of the radially outer surface of the roller of the cam follower to make the rolling or sliding contact with the cam in use, various inventions (as referred to Patent Documents 1 and 2, for example) have been known on the improvements in the surface properties.
In Patent Document 1, an arrangement in which the radially outer surface of the roller is prevented from being peeled, by forming a number of small recesses for oil reservoirs is disclosed.
In Patent Document 2, an arrangement in which the durability is improved to prevent the peeling by enhancing the hardness on the radially outer surface of the roller is disclosed.    Patent Document 1: Japanese Patent No. 2,594,339    Patent Document 2: Japanese Patent No. 3,125,434
However, the arrangements of Patent Documents 1 and 2 could prevent the peeling of the roller, but still have a problem that the cam surface is worn if the cam is made of a material weaker than that of the roller.
In the arrangement of Patent Document 1, specifically, the radially outer surface of the roller is made rough by a machine such as a centrifugal barrel of high working power, so that the roughness becomes seriously coarse. When the roller makes the rolling or sliding contact with the cam, the roughness of its radially outer surface wears the cam surface. At the time of barreling treatment, moreover, the roughness is formed not only on the radially outer surface but also on the radially inner face of the roller. This raises another problem that the member to be contacted by the radially inner face of the roller, such as the rollers of a bearing are worn.
In the arrangement of Patent Document 2, on the other hand, the radially outer surface of the roller is made so drastically hard as to increase the difference in hardness from that of the cam surface. This raises another problem that the cam surface is worn.
The arrangement of Patent Document 1 may be modified such that the radially outer surface is subjected to an SF (Super Finishing) treatment thereby to reduce the surface roughness and to improve the coarseness. However, the axial end faces of the roller are not subjected to the SF treatment so that they are rough. This raises a problem that the inner wall of the holder of the cam follower to be contacted by the axial end faces of the roller is worn out.